The TianQin project: current progress on science

and technology

Jianwei Mei1, Yan-Zheng Bai2, Jiahui Bao1, Enrico Barausse7,

Lin Cai2, Enrico Canuto8, Bin Cao1, Wei-Ming Chen1, Yu

Chen1, Yan-Wei Ding1, Hui-Zong Duan1, Huimin Fan2,

Wen-Fan Feng2, Honglin Fu4, Qing Gao9, TianQuan Gao1,

Yungui Gong2, Xingyu Gou5, Chao-Zheng Gu1, De-Feng Gu1,

Zi-Qi He1, Martin Hendry10, Wei Hong2, Xin-Chun Hu2,

Yi-Ming Hu1, Yuexin Hu3, Shun-Jia Huang1, Xiang-Qing

Huang1, Qinghua Jiang5, Yuan-Ze Jiang1, Yun Jiang1, Zhen

Jiang11,12, Hong-Ming Jin2, Valeriya Korol13, Hong-Yin Li2,

Ming Li1, Ming Li3, Pengcheng Li14, Rongwang Li4, Yuqiang

Li4, Zhu Li1, Zhulian Li4, Zhu-Xi Li2, Yu-Rong Liang2,

Zheng-Cheng Liang2, Fang-Jie Liao1, Shuai Liu1, Yan-Chong

Liu2, Li Liu2, Pei-Bo Liu1, Xuhui Liu5, Yuan Liu1, Xiong-Fei

Lu1, Yang Lu1, Ze-Huang Lu2, Yan Luo1, Zhi-Cai Luo2, Vadim

Milyukov15, Min Ming2, Xiaoyu Pi4, Chenggang Qin2, Shao-Bo

Qu2, Alberto Sesana16, Chenggang Shao2, Changfu Shi1, Wei

Su2, Ding-Yin Tan2, Yujie Tan2, Zhuangbin Tan1, Liang-Cheng

Tu1,2, Bin Wang17, Cheng-Rui Wang2, Fengbin Wang3,

Guan-Fang Wang1, Haitian Wang18, Jian Wang1, Lijiao Wang5,

Panpan Wang2, Xudong Wang5, Yan Wang2, Yi-Fan Wang19,20,

Ran Wei6, Shu-Chao Wu2, Chun-Yu Xiao2, Xiao-Shi Xu1, Chao

Xue1, Fang-Chao Yang2, Liang Yang1, Ming-Lin Yang1,

Shan-Qing Yang1, Bobing Ye1, Hsien-Chi Yeh1, Shenghua Yu12,

Dongsheng Zhai4, Caishi Zhang1, Haitao Zhang4, Jian-dong

Zhang1, Jie Zhang2, Lihua Zhang3, Xin Zhang21, Xuefeng

Zhang1, Hao Zhou2, Ming-Yue Zhou2, Ze-Bing Zhou2,

Dong-Dong Zhu2, Tie-Guang Zi1, Jun Luo1,2,a

1TianQin Research Center for Gravitational Physics & School of Physics and

Astronomy, Sun Yat-sen University (Zhuhai Campus), Zhuhai 519082, P.R. China2Centre for Gravitational Experiments, School of Physics, MOE Key Laboratory of

Fundamental Physical Quantities Measurement & Hubei Key Laboratory of

Gravitation and Quantum Physics, PGMF, Huazhong University of Science and

Technology, Wuhan 430074, P. R. China3DFH Satellite Co., Ltd., Beijing 100094, P.R. China4Yunnan Observatories, Chinese Academy of Sciences, Kunming 650011, China5Beijing Institute of Control Engineering, Beijing 100094, P.R. China

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The TianQin project: current progress on science and technology 2

6Beijing Institute of Spacecraft System Engineering, Beijing 100094, P.R. China7SISSA, Via Bonomea 265, 34136 Trieste, Italy and INFN Sezione di Trieste & IFPU

- Institute for Fundamental Physics of the Universe, Via Beirut 2, 34014 Trieste, Italy8Former Faculty, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129, Torino,

Italy9School of Physical Science and Technology, Southwest University, Chongqing

400715, China10SUPA, School of Physics and Astronomy, University of Glasgow, Glasgow G12

8QQ, UK11National Astronomical Observatories, Chinese Academy of Sciences, Beijing

100012, China12School of Astronomy and Space Science, University of Chinese Academy of

Sciences, Beijing 100049, China13School of Physics and Astronomy, University of Birmingham, Birmingham B15

2TT, United Kingdom14Center for High Energy Physics & Department of Physics and State Key

Laboratory of Nuclear Physics and Technology, Peking University, No.5 Yiheyuan

Rd, Beijing 100871, P.R. China15Lomonosov Moscow State University, Sternberg Astronomical Institute, Moscow

119992, Russia16Dipartimento di Fisica “G. Occhialini”, Universita degli Studi Milano Bicocca,

Piazza della Scienza 3, I-20126 Milano, Italy17School of Aeronautics and Astronautics, Shanghai Jiao Tong University, Shanghai

200240, China18Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210023 &

School of Astronomy and Space Science, University of Science and Technology of

China, Hefei, Anhui 230026, P.R. China19Max-Planck-Institut fur Gravitationsphysik (Albert-Einstein-Institut), D-30167

Hannover, Germany20Leibniz Universitat Hannover, D-30167 Hannover, Germany21Department of Physics, College of Sciences & MOE Key Laboratory of Data

Analytics and Optimization for Smart Industry, Northeastern University, Shenyang

110819, China

E-mail: ajunluo@mail.sysu.edu.cn

Abstract. TianQin is a planned space-based gravitational wave (GW) observatory

consisting of three earth orbiting satellites with an orbital radius of about 105 km.

The satellites will form a equilateral triangle constellation the plane of which is nearly

perpendicular to the ecliptic plane. TianQin aims to detect GWs between 10−4 Hz

and 1 Hz that can be generated by a wide variety of important astrophysical and

cosmological sources, including the inspiral of Galactic ultra-compact binaries, the

inspiral of stellar-mass black hole binaries, extreme mass ratio inspirals, the merger

of massive black hole binaries, and possibly the energetic processes in the very early

universe or exotic sources such as cosmic strings. In order to start science operations

around 2035, a roadmap called the 0123 plan is being used to bring the key technologies

of TianQin to maturity, supported by the construction of a series of research facilities

on the ground. Two major projects of the 0123 plan are being carried out. In this

process, the team has created a new generation 17 cm single-body hollow corner-cube

retro-reflector which has been launched with the QueQiao satellite on 21 May 2018; a

new laser ranging station equipped with a 1.2 m telescope has been constructed and

The TianQin project: current progress on science and technology 3

the station has successfully ranged to all the five retro-reflectors on the Moon; and the

TianQin-1 experimental satellite has been launched on 20 December 2019 and the first

round result shows that the satellite has exceeded all of its mission requirements.

1. Introduction

Major activities in GW detection are expected around 2035. By then the network of

ground-based GW detectors should have detected thousands of GW events and the

pulsar timing array and the cosmic microwave background experiments could have

made historic breakthroughs. Brand new space-based GW observatories, such as LISA

[1] and DECIGO [2, 3], could also join the effort. Proposed in 2014, the TianQin

project aims to launch the space-based GW observatory, TianQin, around 2035 and to

detect GWs between 10−4 Hz and 1 Hz [4]. TianQin is unique in several respects: it

is the only planned detector that uses geocentric orbits [5, 6, 7], like the others it uses

three satellites to form a equilateral triangular constellation, but the constellation plane

is nearly perpendicular to the ecliptic plane, and its frequency sensitivity band is in

between those of LISA and DECIGO, overlapping with that of LISA near 10−4 Hz and

with that of DECIGO near 1 Hz.

As first published in [4], the concept of TianQin envisions a equilateral triangle

constellation of three drag-free satellites orbiting the Earth with an orbital radius of

about 105 km [8, 9, 10], with the detector orientation, i.e. the normal vector to the

constellation plane of the detector, pointing toward RX J0806.3+1527 (also known as

HM Cancri or HM Cnc, hereafter J0806 [11]). The satellites will be carefully controlled

to provide an ultra-clean and stable environment for its scientific operation, allowing

gravity to take full governance of the motion of a set of test masses residing in the

satellites and allowing laser interferometry to reach extremely high precision. In this

way, the variation in the distance between the test masses (partially caused by GWs)

can be measured with the inter-satellite laser interferometry.

Some basic parameters of TianQin are listed in Table 1.

Table 1. Basic parameters of TianQin.

Parameters Value

Type of orbit Geocentric

Number of satellites N=3

Arm length L =√

3× 105 km

Displacement measurement noise S1/2x = 1× 10−12 m/Hz1/2

Residual acceleration noise S1/2a = 1× 10−15 m s−2/Hz1/2

The TianQin project: current progress on science and technology 4

2. Scientific prospects

A systematic effort has been undertaken to assess the expected science output of TianQin

[12, 13, 14, 15, 16, 17, 18, 19, 20]. In these works, the following sensitivity curve has

been used [4, 9, 21],

Sn(f) =10

3L2

[Sx +

4Sa

(2πf)4

(1 +

10−4Hz

f

)]×[1 + 0.6

( ff∗

)2], (1)

where Sx and Sa are given in Table 1, and f∗ = c/(2πL) ≈ 0.28 Hz is the transfer

frequency. Note (1) describes a ballpark goal and the sensitivity curve is expected to

be refined over time. We assume 5 years of mission lifetime for TianQin. With its

baseline concept [4], TianQin is expected to operate in a consecutive “three-month on

+ three-month off” mode, which means that TianQin will firstly observe continuously

for three months, and then be put into a safe mode for the next three months before

starting observation again, in order to cope with the variation of thermal load on the

satellites. In this scheme, the total duration of data acquisition will be of 2.5 years for a

mission lifetime of 5 years. Although various possibilities are being explored to increase

the fraction of total observation time, we use the baseline concept of TianQin to get a

(presumably conservative) assessment of the expected science output.

The major sources expected for TianQin include the inspiral of Galactic ultra-

compact binaries (GCBs), the inspiral of stellar-mass black hole binaries (SBHBs),

extreme mass ratio inspirals (EMRIs), the merger of massive black hole binaries

(MBHBs), and possibly also energetic processes in the very early universe or exotic

sources such as cosmic strings [22]. With the detection of GWs from such sources,

TianQin is expected to provide key information on the astrophysical history of galaxies

and black holes, the dynamics of dense star clusters and galactic centers, the nature

of gravity and black holes, the expansion history of the universe, and possibly also the

fundamental physics related to the energetic processes in the early universe.

A summary of the expected astrophysical sources that can be detected with TianQin

is illustrated in Figure 1.

2.1. Galactic ultra-compact binaries (GCBs)

GCBs are likely the most numerous sources for a space-based GW detector such as

TianQin [23, 24, 25, 26, 17]. The detection of GCBs is of great importance for

astrophysics and fundamental physics. For astrophysics, GW observations are likely

to detect more GCBs with ultra-short periods (< 1 hour) than the electromagnetic

observatories [27], and thus can not only subject the formation theories of GCBs to

precise test but can also help to study the matter distribution and the structure of

the Galaxy [28, 29, 30]. The GWs from GCBs can also be used to study fundamental

physics, such as helping to constrain extra radiation channels or extra polarisation modes

of GWs [31, 32]. It has been shown that TianQin can facilitate studies on the formation

The TianQin project: current progress on science and technology 5

10 4 10 3 10 2 10 1 100

Frequency(Hz)

10 20

10 19

10 18

10 17

10 16

10 15

10 14

Ampl

itude

spec

trum

den

sity(

Hz

1/2 )

J0806

10 5M + 10 5M

10 6M + 10 6M

10 7M + 10 7M

TQ

GCBVBSBHBMBHBEMRIforeground

Figure 1. Expected astrophysical sources for TianQin. In this plot, the information of

the detector response is included in the signal strain of each source and the instrumental

noise (the solid red curve) is approximated by dividing Sn(f) by 10/3, which accounts

for the geometric configuration of TianQin, as well as the location- and polarisation-

sensitive response in the low frequency limit.

of neutron star systems through accretion-induced collapse in white-dwarf binaries [33].

TianQin also has the potential to detect GWs from deformed compact stars [34].

Electromagnetic observation has identified dozens of GCBs that have orbital periods

shorter than a few hours [11, 35, 36, 37, 38, 39]. These GCBs, once detected, can become

very good calibration sources for the detector and they are referred to as verification

binaries (VBs). For example, J0806 has the shortest known period of all GCBs [11].

TianQin can detect J0806 with a signal-to-noise ratio (SNR) of 5 already after two

days of observation. TianQin can also detect 12 VBs, for some of which the GW

amplitude can be determined up to the 5% level if the amplitude and the inclination

angle are not precisely known while other parameters are assumed to be constrained by

electromagnetic studies.

By using a synthetic population of GCBs, we find that TianQin can resolve about

8.7 × 103 GCBs, and for these events, the uncertainties in the parameter estimation

center around: ∆P/P = 31.6 × 10−8, ∆A/A = 0.10, ∆ cos ι = 0.05, and ∆ΩS = 7.94

deg2, where P and A are the period and amplitude of the GW signal, and ι and ΩS

are the inclination angle and the sky localization (the solid angle in which the source is

located) of the source, respectively. Among the GCBs resolvable with TianQin, about

38% can be localized to better than 1 deg2. In the lower frequency range, the incoherent

addition of a vast number of GCBs will form a confusion noise, or sometimes referred

to as a foreground. An order of magnitude estimation shows that TianQin may also

detect one double white dwarf merger event. More detail on the detection of GCBs with

TianQin can be found in [17].

The TianQin project: current progress on science and technology 6

2.2. Stellar-mass black hole binaries (SBHBs)

Another type of quite promising source for TianQin is SBHBs. The discoveries made

by the LIGO Scientific Collaboration and Virgo Collaboration have revealed that there

exist many SBHBs [40]. The merger of SBHBs will generate GWs with frequencies of

order 100Hz. However, years to months before the final merger, the GWs are in the

milli-Hertz regime, making SBHBs interesting sources for TianQin. SBHBs are ideal

objects for multi-band GW detection, which can not only help better detect and measure

the sources themselves but also make the SBHBs powerful laboratories for cosmology

and fundamental physics, as illustrated by their potential use as standard sirens for

cosmology [41] and to greatly improve the constraints on certain parameters of modified

gravity theories [42]. It has been pointed out that TianQin can play an important role

in detecting sources in the deci-Hertz frequency range [43, 44], and that it is possible to

learn about the eccentricity distribution and formation channels by counting the number

of SBHBs observed with TianQin [45]. Some work on the potential of using TianQin to

test general relativity (GR) with GW signals from SBHBs can be found in [46].

There is large uncertainty in the mass distribution of the SBHBs. As many as

five models have been adopted and calibrated by the LIGO Scientific Collaboration and

Virgo Collaboration to study the population properties of SBHBs [47]. The expected

number of detections depends strongly on the SNR threshold. Searching using template

banks requires that the SNR threshold for TianQin is about 12. With this threshold,

there is the possibility that a single digit number of SBHBs events can be detected

with TianQin. With the advancement of data analysis techniques, the requirement on

the SNR threshold may become less stringent by the time when TianQin flies. If the

SNR threshold is lowered to 8, then a single digit number of detections becomes very

probable and there is the possibility that the number of detections can reach a few

dozens. For these events, the probability distribution in redshift peaks at z ∼ 0.05, and

the probability distribution in the logarithm of the total mass peaks at M ∼ 70 M.

Concerning the precision of parameter estimation, the probability distributions of the

(relative) uncertainties in the coalescing time, the sky localization, the chirp mass,

the eccentricity and the luminosity distance peak at ∆tc ∼ 0.1 s, ∆ΩS ∼ 0.1 deg2,

∆M/M ∼ 10−7, ∆e0/e0 ∼ 10−4 and ∆DL/DL ∼ 20%, respectively. For a typical

merger, the error volume is small enough to contain only the host galaxy, while it can

be as small as 2 Mpc3 in the most optimal cases. More detail on the detection of SBHBs

with TianQin can be found in [16].

2.3. Massive black hole binaries (MBHBs)

Most galaxies in the universe host a massive black hole at their centers [48, 49, 50, 51].

The masses of the central black holes are intimately related to the intrinsic parameters

of the corresponding host galaxies. When galaxies collide, some of the massive black

holes form gravitationally bound pairs, i.e., MBHBs, and eventually merge due to the

loss of energy and angular momentum through GW radiation. The mergers of massive

The TianQin project: current progress on science and technology 7

black holes are extremely strong sources of GWs, which travel through the universe for

billions of years but can still preserve significant strength. Some of the signals will be

detected by TianQin. The detection of GWs from the merger of MBHBs can help reveal

the origin and the formation channels of massive black holes, provide a new method to

study the expansion of the universe at high redshift and test various aspects of GR or

the nature of black holes in the strong field regime [15, 52, 14]. With GW signals from

MBHBs, there is the possibility to use TianQin to explore non-singular black holes [53]

and to constrain the mass spectrum and number of axionlike fields by measuring black

hole spins [54].

How the massive black holes are formed throughout the history of the universe is

still under debate. For example, it is unclear whether massive black holes are formed

from the direct collapse of a massive cloud (the heavy seed model) or the remnant of

population III stars (the light seed model) [55], and various models have been proposed

to depict the evolution of massive black holes [56, 57, 58, 59]. By using five different

models for the event rate of MBHBs, it has been found the expected detection rate varies

significantly from one model to another, with the most pessimistic model predicting less

than 0.1 detection per year, and the most optimistic predicting nearly 60 detections

per year, while other intermediate models predict O(1 ∼ 10) detections per year. If

MBHBs with nearly equal component masses of the order 104 ∼ 105 M at redshift 15

are detected with TianQin, then the SNR can reach above 20 and both the luminosity

distance and the chirp mass can be determined at the 10% level. This will enable the

discrimination of different seed models. For relatively low redshift (e.g., z = 2) sources,

for which there is the chance of finding an electromagnetic counterpart, TianQin has

enough sensitivity to detect sources with chirp masses in the range 104 ∼ 106 M 24

hours before the final merger. Such detections can have SNRs as large as 23 and the

sky localization error less than 100 deg2, and so can be used to trigger and guide the

observation of electromagnetic instruments. More detail on the detection of MBHBs

with TianQin can be found in [12, 13].

It is interesting to consider the possible scenario that TianQin is observing at the

same time as another detector, such as LISA. In this case the simultaneous detection

of a massive black hole binary (MBHB) merger signal can significantly improve the

precision of source parameter estimation, such as the three-dimensional localisation as

well as the merger time [60].

2.4. Extreme mass ratio inspirals (EMRIs)

Massive black holes in the center of galaxies in the local universe can be accompanied

by nuclear stellar clusters with sizes of a few parsec and masses up to 107 ∼ 108 M [61].

Compact objects such as stellar mass black holes can sink into the gravitational potential

well through two-body relaxation. If they later become gravitationally bound with the

massive black hole, these compact objects may eventually merge into the massive black

hole through gravitational radiation. The GW signals from such events, called EMRIs,

The TianQin project: current progress on science and technology 8

have distinct features: they can last for hundreds of thousands of cycles and they stop

abruptly if the central objects are really the Kerr black holes as predicted by GR. The

detection of such GW signals with TianQin can precisely map the surrounding geometry

of the central black holes and help test the Kerr hypothesis and GR to exquisite precision

[62]. It has also been shown that the detection of EMRIs with TianQin can be used to

study boson stars [63], to reveal new formation channels of EMRIs through the detection

of their electromagnetic counterpart [64], and to further reveal the dynamics around the

cental massive black hole in our own Galaxy [65].

The rate of EMRIs depends on a variety of astrophysical processes determining

the evolution of massive black holes and the surrounding compact objects. A dozen

population models have been developed by Babak et al. [66]. The calculation of

accurate EMRI waveforms is the most challenging among all GW sources. Despite a

lot of progress, the problem to accurately and efficiently generate full EMRI waveforms

with the effect of self-force is still open [67, 68, 69, 70, 71, 72, 73]. In this regard,

kludge EMRI waveforms have been used to reduced the computational burden [18, 66].

With additional uncertainties in the plunge time of EMRI waveforms, it has been found

that dozens to thousands of EMRIs may be detected with TianQin, with the horizon

distance marked by sources with masses of order 106 M at redshifts near z = 2. For the

parameters that strongly affect the phases of GWs, the relative precision of parameter

estimation is typically of the order 10−6. For other parameters, such as the luminosity

distance and the sky localization, the (relative) uncertainty can be constrained to better

than ∆DL/DL ∼ 10% and to the level ∆ΩS ∼ 10−3 deg2, respectively, for the majority

of sources. The determination of the three dimensional location is precise enough so

that the detected EMRIs can be used as standard sirens for cosmological study [74, 75].

More detail on the detection of EMRIs with TianQin can be found in [18].

2.5. Various cosmological processes

In addition to the late-stage astronomical objects described in the preceding subsections,

many processes related to the (very) early universe can produce a stochastic gravitational

wave background (SGWB).‡ The main generating mechanisms include the amplification

of vacuum fluctuations re-entering the Hubble horizon during inflation, post-inflationary

preheating and related non-perturbative phenomena, first-order phase transitions (PTs)

of the early universe and cosmic defects due to symmetry-breaking of topological

structure (see [78] for a recent review). Recent studies have analyzed the important

consequences of the first-order PTs that are triggered by introducing an effective

operator [79], by the extended Higgs sector [80, 81, 82, 83, 84, 85] as well as by other

exotic TeV scale particles (i.e., axion-like particles [86, 87], heavy neutrinos [88, 89]

and composite resonances [90]) and subsequently have assessed the potential of using

TianQin to detect the SGWB produced from these processes. In some well-motivated

‡ SGWB can also have other origins, such as purely astronomical processes, and even interactions of

massive black holes with ultralight bosons [76, 77].

The TianQin project: current progress on science and technology 9

scenarios, TianQin can detect the SGWB of cosmological origin with an optimal SNR

as high as the order of 105 [19]. So TianQin has the potential to probe energy scales

above the reach of near-future particle accelerators [86, 88, 89, 90] and even up to the

Grand Unified Theories scale [91].

The formation of cosmic string networks during the process of symmetry-breaking

also produces a SGWB, which can be an important observational effect of Grand Unified

Theories [92]. In some idealistic cases, TianQin may be able to probe the general network

of cosmic defects with tensions Gµ & O(10−6) and particularly the cosmic strings with

Gµ & O(10−17), corresponding to the scale of symmetry-breaking at 1016 GeV and

1010 GeV, respectively [19]. Finally, the detectability of secondary GW backgrounds

produced during inflation and from the PBH after inflation has been evaluated in

[93, 94, 95, 96].

Therefore, if any of these GW backgrounds is detected in the future, it will provide

crucial information on the cosmic history and place constraints on the fundamental

theories describing the early universe or on the low-energy effective theories related to

particle physics around the TeV scale. More details of using TianQin to detect SGWB

will be given in [19], which also contains a detailed explanation of how to use the TianQin

sensitivity curve in this regard.

2.6. Cosmology and fundamental physics

GW signals detected with TianQin can be used to fill in gaps in the expansion history of

the universe. Important information can come from detecting SBHBs within a redshift

of order 0.1, EMRIs within a redshift of order 1 and the merger of MBHBs at other high

redshifts. The detection/null detection of primordial GWs can offer valuable information

on the history of the early universe [93, 94, 95]. It has been found that under ideal

circumstances, the observation of massive black hole binary mergers can pinpoint the

Hubble constant to a precision of the order of 10−2 [97, 20].

Every aspect of GWs can be used to test GR. At the stage of generation, GWs from

GCBs are best used for testing the extra radiation channels and extra polarization modes

of GWs§; SBHBs are ideal sources for multi-band observations that can help improve

by orders of magnitudes the constraints on certain parameters describing deviations

from GR [105, 42]; the mergers of MBHBs are ideal sources for testing various post-

Einstein parameters and the Kerr nature of black holes; and EMRIs can be used to study

the surrounding geometry of a massive black hole to high precision. At the propagation

stage, GW signals from far away sources can be used to constrain the dispersion relation

and the speed of GW propagation. As a quantitative analysis of how TianQin can

perform, a test of the black hole no-hair theorem and a study of the constraint on a

particular modified gravity theory using ringdown signals from the merger of MBHBs

have been carried out in [14] and [15], respectively.

§ A devoted study on the extra polarizations of GWs from different gravitational theories and the

corresponding responses and sensitivities of TianQin can be found in [98, 99, 100, 101, 102, 103, 104].

The TianQin project: current progress on science and technology 10

3. Roadmap and technology progress

Inertial reference and inter-satellite laser interferometry are two core technologies of

TianQin, and the corresponding technology requirements, grossly characterised by Sa

and Sx in Table 1, are prerequisites to achieving the scientific goals discussed in the last

section.

As an important foundation to meet such requirements, the study and development

of inertial sensors based on capacitive sensing and electrostatic control have been

underway since 2000 at the Centre for Gravitational Experiments, Huazhong University

of Science and Technology (CGE-HUST). Several flight models have been constructed

and successively tested in orbit [106]. Efforts are being made to further improve on the

performance of the instruments [107, 108]. A study of the effect of space plasma on the

motion of test masses has been carried out in [109]. The development of high precision

laser interferometers has been underway since 2002. A demonstration system of laser

interferometer with 10-m armlength has been built in 2011 [110] and a prototype of

transponder-type inter-satellite laser interferometer has been constructed and tested in

2015 [111, 112]. Further efforts in this direction include the study of the inter-satellite

laser link acquisition [113] and sampling jitter noise of the digital phasemeter [114].

Relativistic effects on the laser propagation and clock comparison have been studied in

[115, 116], while a large-scale passive laser gyroscope aiming to help link the celestial

and terrestrial reference frames is being developed [117, 118].

In order to systematically bring the key technologies of TianQin to maturity, a

technology roadmap called the 0123 plan has been adopted since the beginning of the

TianQin project:

• Step 0: Acquiring the capability to obtain high precision orbit information for

satellites in the TianQin orbit through lunar laser ranging experiments.

• Step 1: Using single satellite missions, where the main goal is to test and

demonstrate the maturity of the inertial reference technology;

• Step 2: Using a mission with a pair of satellites, where the main goal is to test and

demonstrate the maturity of the inter-satellite laser interferometry technology;

• Step 3: Launching a constellation of three satellites to form the space-based GW

observatory, TianQin.

At each step, one to several independent missions/projects are expected, depending on

the progress of technologies and the opportunities of space missions, and the numbers

labelling the steps are the numbers of dedicated satellites which need to be built for

each independent mission/project of the step.

In the following, we report on two major projects, one in step 0 and the other in

step 1, that are being carried out.

The TianQin project: current progress on science and technology 11

3.1. Lunar Laser Ranging (LLR)

In order to help better determine the positions of the TianQin satellites, Lunar Laser

Ranging (LLR) has been an important part of the TianQin project since early on.

Because no dedicated satellite needs to be built for this part of the work, all the projects

related to LLR are categorized as step 0.

A major project on LLR was jointly approved and supported by the China National

Space Administration (CNSA) and the National Natural Science Foundation of China

(NSFC) in 2016. The project mainly involves upgrading/constructing laser ranging

stations on the ground and creating a new generation of corner-cube retro-reflectors

(CCRs) to be installed on the lunar relay satellite, QueQiao, for the Chang’E 4 mission.

Through the project, the LLR station of the Yunnan Observatories in Kunming

has been upgraded and on 22 January 2018 the station became the first in China to

have successfully ranged to the Moon [119]. A new laser ranging station equipped with

a 1.2 m telescope has also been constructed on Fenghuang mountain near the Zhuhai

Campus of Sun Yat-sen University. The station has successfully received laser ranging

signals from all five retro-reflectors on the Moon. A single-body hollow CCR with 17 cm

aperture has also been created and was launched with the QueQiao satellite on 21 May

2018 [120].

3.2. The TianQin-1 Satellite (TQ-1)

The major objectives of TQ-1 include testing the technologies of inertial sensing, micro-

Newton propulsion, drag free control, laser interferometry, temperature control and

center-of-mass measurement with in-orbit experiments.

The preparation for TQ-1 started in 2016 and the project received official approval

from CNSA in 2018. Support for TQ-1 has also been provided by the Ministry

of Education, the Guangdong Provincial Government and the Zhuhai Municipal

Government of the People’s Republic of China. TQ-1 was successfully launched on

20 December 2019 from the Taiyuan Satellite Launch Center in north China’s Shanxi

Province. The satellite completed its startup phase on 21 December 2019 and has been

functioning smoothly since then.

Results show that the satellite has exceeded all of its mission requirements: by

using the inertial sensor, which has a sensitivity of 5× 10−12 m s−2/Hz1/2 at 0.1 Hz , as

the key tool, the acceleration of the satellite has been measured and found to be about

1 × 10−10 m s−2/Hz1/2 at 0.1 Hz and about 5 × 10−11 m s−2/Hz1/2 at 0.05 Hz . The

performance of the micro-Newton thrusters has been evaluated and the thrust resolution

is found to be about 0.1 µN while the thrust noise is found to be about 0.3 µN/Hz1/2 at

0.1 Hz. The residual noise of the satellite after drag free control is measured and found

to be about 3×10−9 m s−2/Hz1/2 at 0.1 Hz , which mainly comes from the micro-Newton

thrusters. The mismatch between the center-of-mass of the satellite and that of the test

mass has also been measured with a precision better than 0.1 mm; the noise level of

the optical readout system is about 30 pm/Hz1/2 at 0.1 Hz ; the temperature stability

The TianQin project: current progress on science and technology 12

at key temperature monitoring positions has been controlled to about ±3 mK per orbit

(about 97.13 min). More details on the performance of TQ-1 can be found in [121].

By providing first hand data on the in-orbit performance of the key payloads that

are essential to the TianQin project, and by narrowing/quantifying the gap between the

current technology capability and the requirement of TianQin, TQ-1 has marked a new

milestone in the development of the TianQin project.

3.3. Other developments

3.3.1. Dedicated research facilities The TianQin project involves constructing a few

dedicated research facilities:

• The TianQin research building: to be used for research and technology development

for the TianQin project. The building has been finished with a total area of more

than 37 thousand square meters and is scheduled to open in 2020.

• The TianQin cave lab: to be used for research and technology development for the

TianQin project. Excavation of the cave lab tunnel has started in 2019 and the

tunnel was holed through on 5 June 2020.

• The Ground Simulation Facility (GSF): to be used for the integrated test and

research on TianQin technologies and prototypes. A pre-study project has been

approved for the construction of this facility.

3.3.2. International collaboration International collaboration is an important aspect

of the TianQin project. There have been 6 International Workshops on the TianQin

Science Mission since 2014. On 18 December 2018, the TianQin Collaboration, including

its international advisory committee, was formally established during the fifth TianQin

workshop.

Acknowledgments

This work was supported in part by the Guangdong Major Project of Basic and

Applied Basic Research (Grant No. 2019B030302001) and the National Natural Science

Foundation of China (Grants No. 11703098, 11805286, 11805287, 11975319, 11654004,

11655001, 41811530087, 11690022). David Blair and Gerhard Heinzel have provided

helpful comments and suggestions when the paper is being prepared.

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